In the extraction from vegetable, oi1 and fat-bearing raw materials, such as oil plants or fruits and oil seeds, with organic, especially polar solvents for the purpose of producing oils, fats, and phosphatides (lecithin), there is obtained, on the one hand, the solution designated as miscella comprising the oils, fats, and phosphatides in solution in the solvents and, on the other hand, a residue free of oil and/or fat or having only a low oil and/or fat content, i.e. the so-called meal.
This residue, which may, for example, still have a residual content of oil of up to 0.5% by weight and contain up to 30% by weight of solvent, after being separated from the miscella by pressing or screening, is treated in a predominantly continuous process with steam or steam-containing fluids and thereby freed from solvent to an optimum degree.
The percentage of solvent in the solvent-wet residue, the meal, is of course dependent on the nature of the solvent or extractant, on the raw material, on the extraction process with its specific process parameters, etc. and may therefore vary within certain limits. However, it is necessary, and that not only for reasons of process economy, to provide for separation of the solvent from the meal and optimum recovery of the solvent; moreover, the meal itself, which is mainly processed to be used as feedstuff for animals and for food products, can only then be utilized if it is passed on for processing in a practically solvent-free condition.
The solvents predominantly used by the respective industry as extractants for oil plants or fruits and oil seeds are aliphatic hydrocarbons, particularly commercial hexane (n-hexane). This hexane has a boiling range of about 63.degree. to 70.degree. C. (145.degree. to 158.degree. F.)--depending on the structure.
It is understood that, depending on the raw material and the extraction process, there are used also other extractants, such as pentanes, heptanes as well as the corresponding iso-compounds or mixtures thereof. However, especially in the production of oil and fat from soy beans, cotton seed, beechnuts, linseed, rape, sesame, sunflowers, etc., the commercial hexane of the above specification has proved to be particularly suitable, so that the invention is described, by way of example, by reference to the extraction from soy material by means of hexane.
As the commercial solvents like hexane etc. are designated by the respective industry as benzenes, the improved process provided by apparatus according to the invention for driving out hexane from the residue of extraction will be designated in the following quite generally and in accordance with the technical terminology as "debenzenizing" (or, rather, "desolventizing", which is the more appropriate term in the English language).
Suitable processes and apparatus for debenzenizing or desolventizing residues of the extraction from oil seeds or oil plants or fruits, the so-called meal, have long been known. Apparatus or devices of this type are offered by the respective industry under the terms toaster, desolventizer, drier or steam cooker in many more or less efficient constructions. Thereby, the meal is freed from extractant with hot air, steam or other suitable fluids in predominantly continuous operation and, at the same time, the extractant is recovered.
Thus, U.S. Pat. No. 2,585,793 describes a toaster developed particularly for extracted soybean flakes wherein the desolventizing is effected by the introduction of direct steam, in the upper region of a multistage system. In the apparatus according to U.S. Pat. No. 2,776,894, the uppermost of a multi-bottom system is provided with steam outlet openings so that the introduced material in the uppermost compartment is directly exposed to the steam. According to German Pat. No. 26 08 712, the direct steam is introduced through the lowermost bottom of the vessel, which is in the form of a perforated or sieve bottom, and is conducted, through likewise perforated compartment bottoms, in opposition to the material, from bottom to top, throughout the entire height of the vessel.
From the publication "Fette-Seifen-Anstrichmittel", 1976, No. 2, pages 56 et seq., there is known "toaster", which is meanwhile used all over the world, in which the solvent-wet meal coming continuously from the extraction, in the uppermost compartment of a multi-compartment system, is treated with direct steam (water vapor), the steam being introduced laterally into the compartment bottom having the form of a double bottom and uniformly distributed, through a plurality of perforations, over the entire cross-section of the bottom. A portion of the steam condenses in the meal, a further portion issues therefrom, together with the solvent, i.e. the benzene, as vapors, and is passed on, as condensate, to a benzene-water separator.
The double bottom of the uppermost compartment--and of each of the further compartments--includes a rotary gate valve through which the desolventized meal continuously falls into the compartment or stage lying below that, the so-called drying stage. By the speed of rotation of the gate valve(s) it is possible to adjust a particular level of material in the individual compartments or stages. The drying stage, which follows the desolventizing, is likewise downwardly defined by a perforated bottom constructed as a double bottom through which hot air for the drying of the meal containing condensate is introduced and uniformly distributed. The air is pressed through the perforated bottom into the meal by means of a ventilator and--laden with moisture--discharged laterally above the bed of meal. Eventually, in the lower stage, the meal is cooled, a ventilator sucking cold air through the meal through a simple perforated bottom.
Regarding the construction, that "toaster-drier-cooler", briefly called "TTK", is a single apparatus, but, with regard to the process, this involves the interaction or the sequence of three operating stages because in each compartment the meal is subjected to a different treatment.
The TTK is especially advantageous economically in view of the low investment costs thereof because three otherwise independent processes with independent apparatus can be performed in a single vessel.
Moreover, due to the vertical succession of the processes, conveying elements, such as transporters, elevators, screw conveyors etc. are dispensed with.
The consumption of energy (steam, hot air, cooling air) is also considerably lower with the TTK compared to a system consisting of three separately operating plants, e.g. a separately operating desolventizer, a fluidized-bed drier connected thereto, and a tubular cooler or the like.
On the other hand, it has been found that especially in the case of TTK's of very large volume, as are used e.g. in the desolventizing of solvent-wet soybean meal, the consumption of electric energy is higher than with known classical desolventizing systems including cascade coolers, multipass driers etc.
Particularly the consumption of electric energy is determined in the TTK substantially by the work performed by the main shaft which carries agitator or rabble arms for each compartment of the multi-compartment system, said rabble arms moving through the meal to convey it uniformly over the compartment bottoms and to prevent the formation of channels due to the upwardly streaming gases (water vapor, hot air, cooling air).
In this connection, tests under service conditions have shown that the main load and consequently the greatest amount of electric energy is involved in the region of desolventizing and, thus, the moving or agitating of the solvent-wet meal, but not in the drying and cooling sector. It is obvious that the wet meal containing up to 30% and more hexane offers greater resistance to the agitator elements than does the material being only water-wet or being already dry. Naturally, electric energy is required also by the ventilators for the movement of hot air and cold air, but this consumption is out of all proportion to that of the drive shaft of the agitator elements in the region of desolventizing proper.
In order to reduce the energy consumption at the main agitator shaft of the TTK, in a further development of the latter, the desolventizing sector itself has been provided with further compartments and, thus, also, further bottoms as well as with rabble arms moving over said bottoms. Furthermore, the height-to-diameter ratio was improved in favor of the overall height, that means, the apparatus was made greater in length but smaller in diameter. Thereby it was made possible to install the bottoms in the desolventizing stage which is always located at the top.
Such a desolventizer-toaster (DT), which is also based on development work by the inventor, is described in EP-A-O 070 496.
In this apparatus, the steam, for desolventizing solvent-wet soybean meal, is introduced as direct steam at the lowest point, i.e. underneath the lowermost bottom of a multi-compartment system, and distributed through perforations. This apparatus is also provided with a central shaft through which the rabble arms are moved slightly above each perforated compartment bottom. The individual bottoms again may be in the form of double bottoms having a plurality of specially designed perforations by which a so-called spray effect is obtained. Thereby the aim is achieved to conduct the steam blown in to the lowermost region of the vertical apparatus on its way upwardly from compartment to compartment unhindered and to practically exclude the formatioh of steam channels.
By this relatively simple construction measure, i.e. by the subdividing of the desolventizing zone into several, particularly three to four compartments, each having an agitator element but all driven through a common shaft, as well as by the provision of a plurality of quite specifically designed perforations or openings in the compartment bottoms, particularly compartment double bottoms, for the passage of steam therethrough, it could be achieved that despite the increase in number of the agitator elements in the desolventizing stage and in spite of the extended path length of the steam streaming from the bottom to the top, the power required at the agitator shaft, with unchanged capacity of the plant, was reduced by 30 to 50%.
Of special importance is also the fact that this apparatus permits a solvent (hexane)/steam vapors temperature of 66.degree. to 68.degree. C. (15O.8.degree. to 154.4.degree. F.) without any losses in regard to the degree of desolventizing, i.e. the residual content of solvent in the meal and the general economy of the plant.
For this reduction of the temperature of the vapors of hexane and steam issuing from the desolventizer, from 72.degree. to 78.degree. C. (161.6.degree. to 172.4.degree. F.) or more, as usual hitherto, to 66.degree. to 68.degree. C. (150.8.degree. to 154.4.degree. F.) and partly less, has the result that the consumption of direct steam for driving out the hexane could be lowered from e.g. 160 to 170 kg per 1,000 kg of hexane-wet soybean meal (a hexane content of about 20%) at over 72.degree. C. (161.6.degree. F.) to slightly more than 150 kg per 1,000 kg of meal at 66.degree. C. (150.8.degree. F.). For present-day type large-scale industrial plants, this is a considerable cost reduction if, for example, several thousands of tons of soybean meal are to be desolventized within short periods of time.
It is assumed that it is particularly the channel formation of steam that is responsible for the higher consumption of steam in the desolventizing stages usual hitherto, without intermediate bottoms, for that additional steam quite certainly leaves the plant unused passing through the material and directly through the connection piece for the vapors.
Therefore, in spite of satisfactory functional performance, it was not always possible with one-stage desolventizers, also with TTK, to operate under conditions that were fully satisfactory economically (steam consumption, optimal expulsion of hexane, economical conduction of process in regard to time, etc.), also not at temperatures in the vapors of 70.degree. C. (158.degree. F.) and higher.
It has been found that even at such low temperatures in the vapors, after a short starting period only, residual hexane contents of 0.05% and over, partly even up to over 1.0%, were proved in the desolventized meal. However, such high hexane contents render the meal unfit for use on grounds of nutrition physiology.
Therefore, one was forced again to revert to conventional temperatures of vapors of over 72.degree. C. (162.degree. F.).
By the installation of a plurality of intermediate bottoms in the desolventizing stage in accordance with EP-A-O 070 496, a continuous redistribution of the upwardly streaming steam takes place from one intermediate bottom to the next, so that the latter is fully utilized practically without the formation of ineffective channels. Moreover, the condensation of steam within the meal is considerably reduced.
Further improvements in regard to operational economy are obtained by a new development devised by the inventor which is described in European Pat. Application No. 82 106 498.7 (Publication No. 0 077 436). In this development, superheated steam at temperatures of up to 215.degree. C. (419.degree. F.) is used. The condensation of steam within the meal is reduced again, which does not only result in reduced steam but also produces a meal having a lower moisture content.
This in turn results in considerable relief of the following drying unit.
The steam consumption is a decisive criterion for the evaluation of an economically conducted desolventization. Added to this are the consumption figures for the motive energy for moving the agitator elements over the compartment bottoms and--as a minor factor only--the motive energy for the fans for the provision of hot air and cooling air.
The moisture content (water content) of the meal increases in the course of the desolventization, the transfer of heat of evaporation to the hexane causing the steam itself to condense. Thus, for example, in the desolventizing of hexane-wet soybean meal, according to a typical evaporation-condensation process close to practical operating conditions, the water content in the meal in the final stage of desolventization rises from about 12% by weight at entry into the desolventization to about 22 to 23% by weight. On the other hand, in the case of soybean, a water content in the desolventized meal of over 17% by weight is quite desirable for reasons of optimal expulsion of hexane, as it is only in this way that the hexane content is reduced to a minimum and the desired increase in nutritional value of the meal is ensured.
This relatively high water content in the desolventized meal does of course require an increased energy input for the drying and this energy input, e.g. for hot air, may indeed be such as to jeopardize the efficiency of the desolventization and, consequently, of the whole oil production plant.